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Tiny traces of carbon trapped inside the oldest diamonds ever found, suggest life started on Earth 700 million years earlier than previously thought, say Australian researchers.

Dr Alexander Nemchin and colleagues from Curtin University's Department of Applied Geology in Western Australia report their findings in this week's issue of Nature.

The team analysed carbon isotopes trapped inside the 4.2-billion year old diamonds that formed inside zircon crystals discovered last year in the Jack Hills region of Western Australian.

Associate Professor Ian Fitzsimons, who heads the department that Nemchin and colleagues belong to, says the ratio of carbon isotopes provides clues on whether the carbon came from a life form.

"When plants take in carbon from the atmosphere they much prefer carbon-12 and don't like carbon-13," says Fitzsimons.

"So the carbon in the plants has a different ratio and anything that eats the plants inherits that ratio."

Nemchin and colleagues found the diamonds had a high concentration of the light carbon-12 isotope, compared to the heavier carbon-13.

"The accepted argument is that must have been biogenic carbon, produced by some process such as photosynthesis," says Fitzsimons.

Assuming the diamonds are older than the zircon they were trapped inside, this would make them the oldest ever discovered.

The oldest evidence for life to date are the 3.5-billion-year-old stromatolites rock structures - believed to have been made by photosynthetic algae - in the Pilbara region of Western Australia, says Fitzsimons.

Some uncertainty

Fitzsimons says it's still possible that the light carbon signatures were formed by inorganic process.

But if this turned out to be the case it would mean theories that rely on the widely held assumption that light carbon means life will be undermined.

"It will upset lots of people and destroy a few theories," says Fitzsimons.

Life from space?

Fitzsimons says there are other implications of life on Earth beginning 4.2 billion years ago.

At 3.8 billion years ago the Earth went was being bombarded heavily by meteorites, one of which is thought to have resulted in the formation of the Moon.

Therefore, Fitzsimons says if Nemchin and colleagues are right, either life was here before the meteorites and survived this violent era, or it was created twice in Earth's history: once before the meteorites, and once after.